Adhesive sheets are used in a wide range of situations in medical fields. Examples thereof include dressing materials and sticking plasters for use in protecting damaged skin surface such as wounded parts and bedsore parts, surgical tapes for use in fixing a catheter, gauze, or the like to a skin surface, and adhesive sheets for transdermal administration of a drug into the body through the skin.
In the case where these medical adhesive sheets are to be applied directly to the skin surface and used in this state, the pressure-sensitive adhesive compositions to be used therein are required to exhibit a skin adhesive force which prevents the adhesive sheets from peeling off even when the underlying skin part moves or perspires in some degree.
On the other hand, when a pressure-sensitive adhesive composition having a too high skin adhesive force is used, there are cases where stripping the medical adhesive sheet off the skin surface not only pulls the underlying skin part to cause an unnecessary pain to the user but also physically strips off the horny layer of the skin to irritate the skin.
Consequently, in the development of a medical adhesive sheet, it is highly important for the pressure-sensitive adhesive composition to reconcile the sufficient retention of a skin adhesive force with reduced skin-irritating properties while attaining a good balance between them.
A technique for regulating these properties has been investigated which comprises conducting a crosslinking treatment, such as a chemical crosslinking treatment using a crosslinking agent or a physical crosslinking treatment using electron beams or ultraviolet, to thereby increase the cohesive force of the pressure-sensitive adhesive layer and incorporating a large amount of a liquid plasticizer into the pressure-sensitive adhesive layer to thereby moderately reduce the skin adhesive force thereof (see, for example, document 1).
However, there are cases where compounds having a functional group, e.g., a percutaneously absorbable drug and an antioxidant, should be incorporated into some medical pressure-sensitive adhesive compositions, and the crosslinking treatment of pressure-sensitive adhesive compositions containing such compounds may cause the following problems.
When a pressure-sensitive adhesive composition to be subjected to a chemical crosslinking treatment contains compounds having a functional group, there are cases where the crosslinking agent reacts with the functional groups of these compounds and, as a result, the effects which in themselves are produced by these compounds, such as a pharmacological effect and an oxidation inhibitive effect, are not produced.
Also, there are cases where the crosslinking agent is consumed by those reactions and the adhesive polymer serving as the basic component of the pressure-sensitive adhesive layer is not sufficiently crosslinked and the cohesive force of the pressure-sensitive adhesive layer does not improve.
In the case of a physical crosslinking treatment, the treatment with electron beams, ultraviolet, or the like for crosslinking may decompose or alter the compounds having a functional group to cause the same problems as in the chemical crosslinking treatment.
Those problems are remarkable especially when the pressure-sensitive adhesive composition contains a percutaneously absorbable drug. This is because most percutaneously absorbable drugs have some functional group in the molecule to exhibit physiological activity.
Various techniques for eliminating those problems have been investigated which comprise incorporating a large amount of a liquid plasticizer into a pressure-sensitive adhesive composition without conducting a crosslinking treatment. Examples of such techniques which have been proposed include a technique in which a pressure-sensitive adhesive composition comprising a methyl vinyl ether/maleic anhydride copolymer having a viscosity in a specific range and a liquid compound having plasticizing action and/or absorption-accelerating action is used (see, for example, document 2) and a technique in which an adhesive polymer having an extremely high weight-average molecular weight is used and both a liquid plasticizer and a solid plasticizer are incorporated therein (see, for example, document 3).
However, the former technique has the following disadvantage. Since a methyl vinyl ether/maleic anhydride copolymer, which is a water-soluble polymer, is used as an adhesive polymer, the pressure-sensitive adhesive layer, during wear, absorbs water such as the sweat of the skin and comes to have a reduced cohesive force. There are cases where stripping off the adhesive sheet after use results in the so-called adhesive remaining on the skin.
On the other hand, in the latter technique, the decrease in cohesive force caused by water absorption described above can be diminished in some degree by selecting a water-insoluble adhesive polymer. However, when a liquid plasticizer is incorporated in an amount exceeding 100 parts by weight per 100 parts by weight of the adhesive polymer, there are cases where the liquid plasticizer and other ingredients cannot be held in the pressure-sensitive adhesive layer.
Although techniques for reducing skin-irritating properties by incorporating a large amount of a liquid plasticizer into a non-crosslinked pressure-sensitive adhesive composition have been known as described above, these are still unsatisfactory. A further improvement is desired.
Document 1: JP-A-2002-212064 (claim 1)
Document 2: JP-A-7-69871 (claim 1)
Document 3: JP-A-2000-26285 (claim 1)